QSettings: support reading UTF-8 keys from INI files

Ini files with unicode characters like German umlauts are not read correctly and saving stores in ASCII. SetIniCodec() was removed and according the documentation it should be UTF8 by default now. Same problem exists on macOS.

#include <QCoreApplication>
 #include <QSettings>
int main(int argc, char *argv[])
{ QCoreApplication a(argc, argv); QSettings settings("test.ini", QSettings::IniFormat); settings.beginGroup("Test"); settings.setValue("BAR",""); settings.setValue("BÄR",""); settings.setValue("OST",""); settings.setValue("ÖSE",""); settings.endGroup(); return a.exec(); }

>file test.ini
test.ini: ASCII text

[Test]
BAR=
B%C4R=
OST=
%D6SE=

Update: the scope of this task is changed to implement only reading support, because writing is not backward-compatible. QTBUG-101380 is created to track the writing support, which might happen in next major release.